The use of ZnO for flue gas cleaning was presented as early as 1940 by Johnstone and Singh. Johnstone, H. F., and Singh, A. D., "Recovery of Sulfur Dioxide from Waste Gases--Regeneration of the Absorbent by Treatment with Zinc Oxide", Ind. Eng. Chem., 32 (8), 1037-1049 (1940). They proposed a type of regenerable double alkali process in which ZnO is reacted with sodium sulfite and bisulfite from an SO.sub.2 absorber to precipitate zinc sulfite. The zinc sulfite is dried and then calcined to give pure SO.sub.2 and ZnO, the latter being recycled.
Lowell, et al. performed a thermodynamic analysis on the oxides of 47 elements for use as sorbents for FGD in processes based upon thermal regeneration of the sorbent. Lowell, P. S., et al., "Selection of Metal Oxides for Removing SO.sub.2 from Flue Gas", Ind Eng. Chem., Process Des. Develop, 10 (3), 384-380 (1971). Under flue gas conditions, thermodynamic analysis predicts that ZnO forms a sulfite in the presence of SO.sub.2 at a temperature of 248.degree. F. and below. Furthermore, the decomposition temperature for zinc sulfite is 374.degree. F., which is lower than for the sulfite of any metal oxide except BeO and Mn.sub.2 O.sub.3. However, zinc sulfite both disproportionates and decomposes to the oxide when heated in an inert atmosphere. The zinc sulfite disproportionates to sulfate and sulfide. Sulfate can also be formed as a result of sulfite oxidation by oxygen present in the flue gas. Zinc sulfate thermally decomposes to the oxide at 1364.degree. F., but with intermediate formation of zinc oxysulfate at 1130.degree. F. In a regenerable flue gas desulphurization process (FGD), it would be highly desirable to suppress the disproportionation and oxidation reactions in order to simplify the regeneration process.
Bienstock and Field studied the reaction between SO.sub.2 and ZnO by passing simulated flue gas (without NO.sub.x or fly ash) through a fixed bed of ZnO at both 265.degree. and 625.degree. F. Bienstock, D., and Field, F. J., "Bench-Scale Investigation on Removing Sulfur Dioxide from Flue Gases", J. Air Pollut. Control Assoc., 10 (2), 121-125 (1960). In both cases, they found that the loading was less than 1 g SO.sub.2 /100 g ZnO when breakthrough of SO.sub.2 occurred. This result is not surprising in view of the thermodynamic analysis which predicts a maximum temperature of 248.degree. F. for zinc sulfite formation. However, DeBerry and Sladek studied the ZnO-SO.sub.2 reaction in the range of 77.degree. to 1472.degree. F. by thermogravimetric analysis; the reaction rate was immeasurably small over the entire temperature range. DeBerry, D. W., and Sladek, K. J., "Rates of Reaction of SO.sub.2 with Metal Oxides", Can. J. Chem. Eng., 49 (6), 781-785 (1971). The composition of their feed gas was 14.3 v/o CO.sub.2, 3.4 v/o O.sub.2, 2.0 v/o H.sub.2 O, 0.10 to 0.35 v/o SO.sub.2, and balance N.sub.2. Their H.sub.2 O concentration was considerably lower than in actual flue gas.
Graefe, et al. studied the ZnO-SO.sub.2 reaction at 131.degree. F. with simulated flue gas saturated with H.sub.2 O and obtained SO.sub.2 loadings of about 30 g/100 g ZnO. Graefe, A. F., et al., "The Development of New and/or Improved Aqueous Processes for Removing SO.sub.2 from Flue Gas", Vol. II, Envirogenics report to NAPCA, PB 196 781 (October, 1970). Their most interesting conclusions are that water vapor is required for absorption to occur, high humidities give higher absorptions, and a liquid water phase is not required. It was not stated how the reaction products were identified, but the main product was reported to be ZnSO.sub.3. 21/2H.sub.2 O. Some zinc sulfate was formed mainly because of the presence of NO.sub.2. Graefe, et al. also found that zinc sulfite can be decomposed below 572.degree. F. without disproportionation and without oxidation in the absence of air and preferably in the presence of steam. They found that zinc sulfate can be decomposed at 1832.degree. F. Graefe, et al. proposed a ZnO fluidized bed system for the absorption and regeneration of SO.sub.2, with separate thermal regenerators for sulfite and sulfate.
In Japan, Mitsui Mining and Smelting Company, Ltd., has developed a ZnO-based SO.sub.2 scrubbing system that is in commercial use at their electrolytic zinc plants. An aqueous slurry of ZnO is fed to an absorption tower for reaction with SO.sub.2 in tail gas from an acid plant to form zinc sulfite and bisulfite. A bleed stream from the tower is reacted with sulfuric acid to produce zinc sulfate and SO.sub.2. The SO.sub.2 gas is returned to the acid plant and the zinc sulfate solution is used at the ore roasting plant or for the production of zinc sulfate.
Several papers discuss the use of ZnO as a catalyst to reduce or decompose NO. Kortum and Knehr found that activated ZnO is capable of reducing NO at room temperature to N.sub.2 O. Kortum, V. G., and Knehr, H., Reflexionsspektroskopische Untersuchungen im IR uber die Adsorption von NO an Zinkoxid", Berichte der BunsenGesellschaft, 77 (2), 85-90 (1973). Yur'eva, et al. found that ZnO catalytically decomposes NO in the range of 1200.degree. to 1380.degree. F. Yur'eva, J. M., et al., "Catalytic Properties of Metal Oxides of Period IV of the Periodic System with Respect to Oxidation Reactions - II. Decomposition of Nitric Oxide", translated from Kinetika i Kataliz, 6 (6), 1041-1045 (1965). Alkhazov, et al. found that ZnO can act as a catalyst for the reduction of NO by CO in the range of 210.degree. to 930.degree. F. Alkhazov, F. G., et al., "Catalytic Activity of Transition-Metal Oxides for the Reaction of Nitric Oxide with Carbon Monoxide", translated from Kinetika i Kataliz, 16(5), 1230-1233 (1975).
Although a conceptualized flue gas desulphurization process using ZnO has been described in the literature by Graefe et al., it is not the optimum process. A fluidized bed reactor would not be the best contacting device for flue gas subsaturated with water. In addition, there is a complete lack of data on the reaction between ZnO and SO.sub.2 from flue gas in the temperature range of most interest (150.degree. to 250.degree. F.). Operation in this temperature range corresponds to flue gas subsaturated with water and would allow the use of contacting devices such as a spray dryer and/or a fabric filter or a partial gas quench followed by a bed filter. Also, it would be possible to eliminate the need for a separate sulfate regenerator as described later.
Previous work on the use of ZnO for combined SO.sub.2 /NO.sub.x removal from flue gas could not be found in the literature, but the possibility was alluded to by Graefe, et al. Data on the reaction between ZnO and NO.sub.x from flue gas could not be found for any temperature range.
Currently, the leading flue gas desulfurization process in terms of installed utility capacity is wet limestone scrubbing. The process employs a limestone slurry to contact the flue gas and react with the SO.sub.2 to produce a calcium sulfite/sulfate sludge for waste disposal. In addition to the waste disposal requirements, process reliability is still a problem area. Plugging and scaling in the system cause a high amount of maintenance work and corrosion can be severe. It is still difficult to select an adequate material to withstand the conditions in the outlet duct and stack. The maintenance and materials problems can add significantly to the process costs. Also, the process does not have any NO.sub.x removal capability, and with high-sulfur coal, it is difficult to achieve the 90 percent SO.sub.2 removal required by the New Source Performance Standards. Alternatives for increasing the SO.sub.2 removal include the use of additives to enhance the limestone reactivity, or the use of slaked lime which is more reactive than limestone. These alternatives add to the process cost.
The need for FGD processes that are more reliable, environmentally acceptable, and less costly has led to the development of the spray dry scrubbing process. Spray nozzles or centrifugal atomizers in a spray dryer create a mist of fine droplets of slaked lime slurry (limestone does not possess sufficient reactivity) into which the SO.sub.2 is absorbed as the flue gas mixes intimately with the spray. Water in the droplets is evaporated by the sensible heat in the flue gas, so that the calcium sulfite and sulfate reaction products leave the scrubber as a dry powder entrained in the flue gas. The reaction products are collected along with the fly ash in a fabric filter or electrostatic precipitator following the spray dryer. The quantity of water fed to the spray dryer is regulated so that evaporation does not cool the flue gas closer than about 20.degree. F. to its adiabatic saturation temperature (typically about 125.degree. F.). Because the gas remains unsaturated and the slurry pumping requirements are greatly reduced (all of the water is evaporated rather than being recycled), many of the operating problems associated with wet scrubbers are avoided.
However, there is still a dry waste product for disposal and lime, rather than less expensive limestone, must be used as the sorbent. Also, it is difficult to achieve 90 percent SO.sub.2 removal with high-sulfur coal unless the lime feed rate is increased to the point where the cost may become prohibitive. The NO.sub.x removal is nil; in fact there is considerable evidence that a visible plume is produced at certain temperatures because NO is oxidized to NO.sub.2 as it passes through the solids layer in a fabric filter. The process can be modified to achieve 50 to 70 percent NO.sub.x removal by using sodium hydroxide as an additive and operating at a spray dryer outlet temperature of 200.degree. F. (75.degree. F. above saturation). However, sodium hydroxide is expensive and the waste product contains soluble salts that can cause surface and groundwater contamination.
Thus far, NO.sub.x emissions from stationary sources have not received as much attention as SO.sub.2 emissions. It is possible to meet the current EPA regulations for NO.sub.x emissions through combustion modifications. However, NO.sub.x control will probably receive increased attention in the future. NO.sub.x emissions are believed to have a deleterious effect on human health and visibility and are believed to be significant contributors to the formation of both nitrate and sulfate acid precipitation. Furthermore, the total annual NO.sub.x emission from stationary sources has been steadily increasing, while SO.sub.2 and particulate emissions have been leveling off or decreasing.
The most advanced technology for NO.sub.x removal from flue gas is selective catalytic reduction (SCR) with ammonia. This process operates at 700.degree. F. and, therefore, is difficult to retrofit. In Japan, flue gas cleaning on a coal-fired boiler includes an SCR system for NO.sub.x removal, an electrostatic precipitator for fly ash removal, and a wet limestone FGD system for SO.sub.2 removal. Since the combined cost of the separate processes for SO.sub.2 and NO.sub.x control is relatively high, there is incentive for the development of a process for the simultaneous removal of SO.sub.2 and NO.sub.x. If such a process were regenerable, i.e., did not produce a waste product for disposal, it would be even more attractive.
Several other processes for the simultaneous removal of SO.sub.2 and NO.sub.x from flue gas have been proposed and tested, but none of them have achieved commercial significance. These processes include the use of activated carbon, copper oxide, or sodium aluminate as dry sorbents, the use of electron beam radiation in conjunction with spray dry scrubbing, and the use of iron sulfide (pyrites) in a wet scrubber. Each of these processes has disadvantages that are absent from the ZnO process. The activated carbon process requires ammonia for NO.sub.x removal, involves the circulation of a large inventory of solids, and requires thermal regeneration at high temperature. The copper oxide process also requires ammonia for NO.sub.x removal and a reducing gas such as hydrogen for conversion of copper sulfate to copper and SO.sub.2 ; it is not suitable for retrofit because the operating temperature is about 700.degree. F. as compared with a 300.degree. F. flue gas exit temperature from the boiler train. The sodium aluminate process requires a reducing gas for regeneration and a gas-solid contacting device that would have a relatively high pressure drop. The electron beam process requires a source of high energy electrons and uses either ammonia, which is converted to fertilizer, or lime, which produces solid waste, including water-soluble calcium nitrate. The chemistry of wet scrubbing with pyrites is extremely complex and the process requires thermal regeneration at high temperature.
In contrast, the ZnO process requires no raw materials other than makeup ZnO, produces no waste product, uses conventional equipment currently in operation on utility FGD systems, and requires moderate temperatures for thermal regeneration. For these reasons, the ZnO process should cost substantially less than the other simultaneous removal processes. The cost of the ZnO process is best compared with the combined cost of SCR and wet limestone FGD for application on high-sulfur coal-fired boilers. However, it must be remembered that the latter more conventional scheme produces solid waste for disposal; hence, power plants in urban locations may not be able to utilize a non-regenerable FGD process, or will have to incur substantial costs over those estimated for disposal.
The current invention describes a regenerable process for the simultaneous removal of SO.sub.2 and NO.sub.x from waste gas streams. Further, the range of conditions for the efficient simultaneous removal of these compounds has been discovered that makes the process amenable to widespread use.